The present invention relates to a road surface condition determination apparatus for a vehicle, which accurately detects the condition of a road, in particular, a rough road including a bumpy road.
Recent vehicles carry a controller which estimates parameters indicating the condition of a road surface such as a road surface frictional coefficient and which performs accurate control of vehicle behaviors such as control of four wheel driving and automatic brake control using the estimated parameters.
It is therefore important to estimate the parameters indicating the condition of a road surface such as a road surface frictional coefficient accurately in order to perform accurate control over various behaviors of a vehicle. Since it is difficult to estimate a road surface frictional coefficient when the road is rough, the detection of roughness of a road surface is an important technique in recognizing the condition of the road surface.
For example, according to JP-A-11-230867, an examination is carried out on a front wheel at which a road surface frictional coefficient has been calculated as a low μ attributable to icing to see whether the calculated value of a road surface frictional coefficient originates from a slip on a bumpy or rough road by comparing the slip ratio at the time of calculation of the road surface frictional coefficient and the slip ratio of the rear wheel on the same side at the same slipping position as the front wheel to see any change in the ratio.
However, the technique for identifying a roughness on a road surface according to JP-A-11-230867 has a problem in that it requires a long checking time because a roughness cannot be identified until the slip ratio of a rear wheel increases after an increase in the slip ratio of a front wheel. For example, when a vehicle having a wheel base of about 2.6 m travels at a speed of 15 km/h, such identification takes about 620 m(sec), which makes it difficult to perform control with high response. According to JP-A-11-230867, since the slip ratio of a front wheel and the slip ratio of a rear wheel are compared, a slip of wheels on a road having a low μ can be erroneously determined as attributable to a rough road.